This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 10-005043, filed Jan. 13, 1998, the entire contents of which are incorporated herein by reference.
The present invention generally relates to a generating system interconnected to a power system. More particularly, it relates to a stabilizing apparatus to be incorporated into the magnetic excitation control system of a rotating type generator such as an alternator and designed to attenuate power fluctuations and enhance the stability of the power system.
Magnetic excitation systems for exciting field circuits of generators such as alternators that are rotating type generators can be generally and roughly classified into AC excitation systems, DC excitation systems and static excitation systems. The AC excitation system uses an AC exciter. The DC excitation system uses a DC exciter. The static excitation system uses a semiconductor switching element such as a thyristor.
A thyristor excitation system, which is a typical static excitation system being popularly used at present as excitation system, will be described below. Also, a power stabilizing system (PSS) adapted to be used in a thyristor excitation system will be explained.
FIG. 1 is a block diagram of an excitation system using a conventional PSS that can effectively attenuate power fluctuations of generator mode (power fluctuations of a short cycle of about 1 to 2 seconds).
As shown in FIG. 1, the generator excitation control system receives an input an AVR reference voltage 2 (hereinafter referred to as xe2x80x9c90ORxe2x80x9d) and the output of transformer 3 (hereinafter referred to as xe2x80x9cPTxe2x80x9d) operating an instrument to an automatic voltage regulator 4 (hereinafter referred to as xe2x80x9cAVRxe2x80x9d) in order to maintain at a constant value the terminal voltage of generator 1 connected to a turbine T. The AVR reference voltage 2 serves to establish a generator voltage. The potential transformer 3 detects the generator voltage. The AVR 4 operates to control the generator voltage.
PSS 5 is provided to produce the generator 1 operate on a stable basis. The output signal of the PSS is input to the AVR 4 and used in the operation of controlling the generator voltage. The field voltage of the generator 1 is thereby regulated to control the transient active power of the generator 1 in order to suppress power fluctuations.
An excitation transformer 6 is arranged to get an excitation source out of the voltage of the generator 1. The output voltage of the excitation transformer 6 is input to a thyristor bridge 7. The field voltage of the generator 1 is modified to regulate the generator voltage according to the value set by said 90 R 2 by controlling the ignition angle of the thyrister bridge 7.
The PSS 5, which is currently commercially available, detects the active power P8 of the generator 1 from the generator voltage detected by the PT 3 and the generator current detected by the CT. The PSS 5 then detects and calculates a change xcex94P in the active power P8, a change xcex94xcfx89 in the rotational speed xcfx899 of the rotor of the generator 1, or a change xcex94f in the generator voltage frequency corresponding to the change in the system side frequency (not shown). The PSS may use one of these signals or two or more of the signals (hereinafter referred to as xe2x80x9cmultivariable PSSxe2x80x9d).
Of multi-variable PSSs, those of the type that use the change xcex94P in the active power of the generator 1 as input and have an appropriate stabilization function (hereinafter referred to as xe2x80x9cxcex94P-PSSxe2x80x9d) are most widely used at present.
The reason for this is that the change in the active power of the generator can be electrically detected and a stabilization function can be set into the PSS with ease because the PSS does not require phase compensation as much as a PSS (hereinafter referred to as xe2x80x9cxcex94xcfx89-PSSxe2x80x9d) that uses the change xcex94xcfx89 in the rotational speed xcfx89 9 of the rotor of the generator 1 as input, although the latter also has an appropriate stabilization function.
The multi-variable PSS 5 shown in FIG. 1 is a typical PSS adapted to cover a broader frequency band subject to power fluctuations than a xcex94P-PSS and a xcex94xcfx89-PSS as it comprises both a xcex94P-PSS and a xcex94xcfx89-PSS that can effectively suppress power fluctuations. This is why such a multi-variable PSS (hereinafter referred to as xe2x80x9c(xcex94P+xcex94xcfx89)-PSSxe2x80x9d is used for a thyristor excitation system.
There are PSSs of other types that may also be used for thyrsitor excitation systems, including one (hereinafter referred to as xe2x80x9cxcex94f-PSSxe2x80x9d) that uses a frequency signal representing either the voltage or the current of the generator 1 as input and also has an appropriate stabilization function) and one (hereinafter referred to as xe2x80x9c(xcex94P+xcex94xcfx89)-PSSxe2x80x9d) that comprises both a xcex94P-PSS and a xcex94xcfx89-PSS.
Various PSSs as described above may also be used for AC/DC excitation systems.
The excitation system further comprises an excessive-excitation limiting device for preventing excessive excitation of the generator 1, an inadequate-excitation limiting device for limiting inadequate excitation of the generator 1, a V/F controlling device for excessive excitation of the exciting transformer 6 or the armature winding of the generator 1, and the like, where V represents the generator voltage and F represents the generator frequency.) However, these devices do not exert any direct influence on the operation of the PSS 5 and, therefore, only the AVR 4 and the PSS 5 are discussed here in detail.
Both analog hardware and digital hardware are commercially available. The AVR 4 and the multi-variable PSS 5 are applicable to hardware of either type in functional terms.
While various types of excitation systems are available as pointed out above, the one shown in FIG. 1 is of the type that is mainly used at present. Therefore, the prior art technologies will be discussed below by way of this excitation system.
FIG. 2 is a block diagram of a conventional AVR 4, illustrating its configuration. Referring to FIG. 2, PSS output signal 5A of multi-variable PSS 5 is input to the AVR 4. Adder A1 adds the PSS output signal 5A to the outcome of the computation for determining the deviation of the generator voltage Vg3A as detected by PT3 from the 90R 2. The signal xcex94V70 obtained as a result of the addition is input to a voltage control section 11 operating on the basis of a gain and an advance/delay to be used to stabilize the voltage control loop.
The output of the voltage control section 11 is equivalent to the field voltage Efd 12 of the generator 1.
FIG. 3 is a schematic block diagram of a known multi-variable PSS 5. As shown in FIG. 3, the change xe2x88x92xcex94P in the active power is made to pass through a stabilization function Gp(S) 13, while the change Aco 9A in the rotational speed xcfx89 9 of the generator 1 is made to pass through a stabilization function Gw(S) 14 before they are added by adder A2. The sum of the addition is input from an output limiter 15 to the AVR 4 as PSS output signal 5A. The stabilization functions Gp(S) 13 and G(w) 14 can remove noise from the input signal by passing the latter through a reset filter 16, an advance/delay circuit 17 and a limiter 18, as shown in FIG. 4.
Due to the above described functional features, the multi-variable PSS 5 can eliminate any steady-state deviations for AVR control that arises when no power fluctuation occurs and correct the phase to output an appropriate voltage regulating signal.
Meanwhile, in recent years, the stability of power system is threatened than ever as the power system increases in scale. As a result, there occur not only local fluctuations that have been a main problem and are short-cycle fluctuations lasting for about 1 second but also inter-system fluctuations that are long-cycle fluctuations lasting for about 2 to 3 seconds.
The xcex94P-PSS that is used in many generators in service at present effectively suppresses the local fluctuation.
The (xcex94P+xcex94xcfx89)-PSS is also used in many generators to suppress the long-cycle power fluctuation. It is reported that the (xcex94P+xcex94xcfx89)-PSS effectively raises the level of power that can be supplied on a stable basis. (See xe2x80x9cDevelopment of Pulse PSS for Suppressing Power Fluctuations in a Broad Area,xe2x80x9d Meeting of Power and Energy Department, Society of Electricity, 1996, xe2x80x9cDevelopment of Multi-PSS for Suppressing Long-Cycle Fluctuation in Interconnected Systems,xe2x80x9d Theory of Electricity B, Voltage. 115-B, No. 1, 1995.) As the amount of interchange power increases among power companies, the cycle period of long-cycle power fluctuations increases and it has become difficult for the existing PSS to suppress long-cycle power fluctuations.
Efforts are being made to interchange power among power companies to an enhanced extent in order to increase the efficiency of operation of the power plants of power companies and run the associated systems more flexibly. It is planned to interchange more power among power companies in the future. Further, as the sales of power to remote customers by IPPs (Independent Power Plants) rises along with the self delivery of power from house generators, power will be supplied over long distances on a huge scale.
Let us imagine, for example, power systems 68A, 68B and 69, each comprising a plurality of generators G and a load, may be interconnected by power transmission lines 60A and 60B. Then, power may be supplied from the power system 68A to the power system 68B over a long distance through the lines 60A and 60B.
With such an arrangement, more power will be expectedly supplied from the power system 68A to the power system 68B than ever within a period of several years from now.
When this expected increase in the amount of power interchanged among power companies is taken into consideration, it seems difficult for the (xcex94P+xcex94xcfx89)-PSS to maintain the stability of the power systems when large scale power fluctuations result from a severe accident such as three-phase earth fault induced by thunderbolt. Then, it will not possible for the (xcex94P+xcex94xcfx89)-PSS to control power fluctuations of generator mode developing in the power systems 68A, 68B and 69. Nor will it be possible for the (xcex94P+xcex94xcfx89)-PSS to control power fluctuations of system mode developing between the power systems 68A and 68B. In other words, the limit to interchange power is defined by the limit to which the stability of the power systems can be maintained.
FIG. 6 is a graph illustrating the outcome of a stability simulation conducted on the assumption that an accident occurred as a result of a three-phase earth fault of a power system providing service over a broad area, involving long-distance power transmission. The simulation shows the waveform of fluctuating power that appears after the accident if a known PSS is used. The cycle of fluctuation of power caused by the accident is about 5.5 seconds. The power fluctuation remains even 40 seconds after the accident to prove it is almost getting to the stability limit. If the level of interchange power is raised under this condition, the power systems will no longer be able to secure its stability.
As pointed out above, it is known that fluctuating power occurs in generator mode and also system mode. More specifically, fluctuating power can appear in generator mode among the generators of a same power company with a cycle of about 1 second and also in system mode among the generators of different power companies with a long cycle (of about 2 to 10 seconds). Thus, it is necessary to develop a new large PSS that can effectively suppress power fluctuations in both modes.
While the xcex94P-PSS that uses a change xcex94P in the active power of a generator as a stabilizing signal is incorporated in many plants at present, it is theoretically adapted to suppress power fluctuations that last for about 1 second or less (between 0.5 seconds and 1 second).
However, it can hardly suppress long-lasting system-mode power fluctuations that continue for about 2 to 10 seconds.
On the other hand, the xcex94xcfx89-PSS that uses a change xcex94xcfx89 in the rotational speed of the rotor of the generator 1 as a stabilizing signal can effectively suppress long-lasting system-mode power fluctuations that continue for about 2 seconds.
The xcex94f-PSS that uses a change xcex94f in the frequency as a stabilizing signal tends to operate almost in the same way as the xcex94xcfx89-PSS.
At present, a combination of the xcex94P-PSS and the xcex94xcfx89-PSS, i.e., the (xcex94P+xcex94xcfx89)-PSS, is employed for the purpose of suppressing power fluctuations that last for about 0.5 seconds to about 2 seconds. Actually, this system works effectively.
However, as more power is interchanged among power companies, long-lasting power fluctuations that continue for about 2 seconds or more occur more frequently and the period of power fluctuations becomes longer as a function of the amount of interchange power. The (xcex94P+xcex94xcfx89)-PSS can suppress power fluctuations lasting for about 2 second or more only with low efficiency.
The exciting systems for generators that operate together with power systems, are roughly classified into two types, i.e., static excitation system and rotary exciting system. The thyristor exciting system is a typical static exciting system, whereas AC exciter is a typically rotary exciting system.
The object of the present invention is to provide a PSS that can quickly suppress power fluctuations that may usually occur over a broad cycle zone, ranging from fluctuations of generator mode (having a short cycle of about 0.5 seconds) to fluctuations of system mode (having a long cycle of about 10 seconds), in order to stabilize power systems and secure power interchange over a broad area on a stable basis and is applicable to both a static exciting system and a rotary exciting system, without adversely affecting the shaft-twisting vibration of the turbines or generators.
The above object of the present invention is achieved by providing a generating system having a rotating type generator to interconnect a power system in order to output power of the generator to the power system, the generating system comprising:
an exciting circuit for exciting the field circuit of the generator;
an excitation control section for controlling the excitation of the exciting circuit in order to regulate the output of the generator;
a short-cycle stabilizing section for outputting a short-cycle stabilizing signal for suppressing short-cycle power fluctuations in accordance with at least one of an electric parameter and a mechanical parameter of the generator;
a long-cycle stabilizing section for outputting a long-cycle stabilizing signal for suppressing long-cycle cycle power fluctuations having a cycle time longer than short-cycle power fluctuations in accordance with the mechanical parameter of the generator; and
an output section for outputting the output of the short-cycle stabilizing section and that of the long-cycle cycle stabilizing section to the excitation control section.
In another aspect of the invention, there is also provided an apparatus for stabilizing a power system to be incorporated into the magnetic excitation control system of a rotating type generator in order to quickly attenuate power fluctuations and enhance the stability of the power system, the apparatus comprising:
a short-cycle stabilizing section for computationally determining a short-cycle stabilizing signal for suppressing short-cycle power fluctuations in accordance with at least one of an electric parameter and a mechanical parameter of the generator;
a long-cycle stabilizing section for computationally tationally determining a long-cycle stabilizing signal for suppressing long-cycle power fluctuations having a cycle time longer than short-cycle power fluctuations according to the mechanical parameter of the generator; and
an adding section for applying the output of the short-cycle stabilizing section and that of the long-cycle stabilizing section to the magnetic excitation control system.
While a rotating type generator according to the invention may typically be an alternator that can normally be used for a hydraulic power system, a thermal power system or an atomic power system, it can also be used for a generator-motor or an induction generator having distributed winding that is designed to be applicable to a pumping-up power system.
The electric parameter of the generator to be used for generating a short-cycle stabilizing signal and/or a long-cycle stabilizing signal may be an active power signal of the generator, a voltage signal of the generator or a signal equivalent to it, a current signal of the generator or a signal equivalent to it, a voltage frequency signal of the generator or a signal equivalent to it or a current frequency signal of the generator or a signal equivalent to it.
The mechanical parameter of the generator may be a rotational speed signal of the rotor of the generator or a signal equivalent to it, a phase angle signal of the rotor of the generator or a signal equivalent to it, an guide vane opening signal of the water wheel linked to the generator or a valve opening signal of the turbine coupled to the generator.
Thus, according to the invention, it is now possible to secure power interchange over a broad area on a stable basis by quickly suppressing power fluctuations that may usually occur over a broad cycle zone, ranging from short-cycle fluctuations (of generator mode) to long-cycle fluctuations (of system mode), thereby stabilizing power systems.
In still another aspect of the invention, there is also provided a generating system having a rotating type generator to interconnect a power system in order to output power of the generator to the power system, the generating system comprising:
an exciting circuit for exciting the field circuit of the generator;
an excitation control section for controlling the excitation of the exciting circuit in order to regulate the output of the generator;
a long-cycle stabilizing section for outputting a long-cycle stabilizing signal for suppressing long-cycle power fluctuations having a cycle time longer than short-cycle power fluctuations in accordance with a mechanical parameter of the generator; and
an output section for outputting the output of the long-cycle stabilizing section to the excitation control section.
In still another aspect of the invention, there is also provided an apparatus for stabilizing a power system to be incorporated into the magnetic excitation control system of a rotating type generator in order to quickly attenuate power fluctuations and enhance the stability of the power system, the apparatus comprising:
a long-cycle stabilizing section for computationally determining a long-cycle stabilizing signal for suppressing long-cycle power fluctuations according to the mechanical parameter of the generator.
Thus, according to the invention, it is now possible to secure power interchange on a stable basis by quickly suppressing long-cycle power fluctuations (of system mode), thereby stabilizing power systems. Differently stated, the generator mode practically does not give rise to any problem when there is no adjacently located generator or when a plurality of generators transmit power to a remote load by way of a system impedance so that it is only necessary to suppress power fluctuations of system mode. Then, the above described arrangement of PSS can effectively suppress power fluctuations of system mode as it comprises a long-cycle stabilizing section for computationally determining a long-cycle stabilizing signal.
While a rotating type generator according to the invention may typically be an alternator that can normally be used for a hydraulic power system, a thermal power system or an atomic power system, it can also be used for a generator-motor or an induction generator having distributed winding that is designed to be applicable to a pumping-up power system.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.